There are many situations where it is important to stabilize sloping or non-sloping ground. Natural slopes such as those on hillsides or created by excavating highway grades may be temporarily stable but then become unstable with time as a result of, for example, material weathering, changes in moisture content, and increases in ground water pressures. Unstable slopes may be created during certain types of construction, such as freeway widening, golf course construction, or other types of construction where the ground is altered. These slopes may be naturally occurring or they may be the result of human activity. These slopes often need stabilization, even when there are no immediate signs of slope failure.
Similarly, it may be desirable for safety reasons, to strengthen certain slopes that are relatively stable, whether naturally occurring or the result of human activity. For example, it is prudent to stabilize slopes behind power plants, or slopes at the base of dams or bridges, even when the slope does not appear to be at or near failure. Also, non-sloping ground adjacent to water may benefit from stabilization.
Most of the research and work on slope and ground stabilization relates to stabilizing landslides. Research on mitigation techniques for shallow, colluvial landslides has seen some interest from the geotechnical community in the past 20 years, although most research has been performed on the predictive analysis of these types of slides (e.g., Aubeny and Lytton, 2004; Cho and Lee, 2002; Collins and Znidarcic, 2004; Iverson, 2000). Predictive analysis techniques are an important aspect of understanding slope stability behavior. Existing methods of landslide mitigation have also been summarized by Rogers (1992). They include excavation and recompaction, conventional retention structures, subdrainage, soil reinforcement using geomembranes and geosynthetics, mechanically stabilized embankments, and combination mechanically stabilized retention structures. However, it may be desirable to stabilize ground or slopes, even when there is no direct prediction of failure, for safety reasons.
Others, such as Ito et al. (1981, 1982), have addressed rotational landslides. These deep landslides have been mitigated with extremely long (25-100 feet (7.6-30.5 meters)) columns (piles) placed in a portion of the potential slide area, generally at the toe of the slope to lock down the base of the potential slide. However, these long, heavy piles are often prohibitively expensive.
Patents have issued describing some of the above-mentioned techniques. Devices and techniques for large scale slope stabilization are described in U.S. Pat. No. 2,880,588 issued to Moore, U.S. Pat. No. 5,797,706 issued to Segrestin et al., German Patent No. DE 4226067 issued to Hermann, and Japanese Patent No. JP 57071931 issued to Yoshihisa. However, these large-scale retaining walls require the use of heavy equipment, and are unsuitable for stabilizing smaller, less accessible slopes.
Other patents deal with stabilizing soil that is adjacent to water, for example U.S. Pat. No. 1,073,278 issued to Mosher, U.S. Pat. No. 3,412,561 issued to Reid, and U.S. Pat. No. 6,659,686 issued to Veazey. However, these patents do not specifically address slope stabilization of shallow landslides.
Still other patents describe the use of posts or anchors. See, e.g., U.S. Pat. No. 1,408,332 issued to Zimmerman, U.S. Pat. No. 1,433,621 issued to Hutton, U.S. Pat. No. 4,530,190 issued to Goodman, U.S. Pat. No. 1,109,020 issued to Skiff et al., U.S. Pat. No. 6,666,625 issued to Thornton, U.S. Pat. Nos. 7,090,440 and 7,811,032 to Short, and German Patent No. D334,121 issued to Van Handel III.
Unfortunately, most of these mitigation options are often not useful, mainly due to economic considerations. Retention structures, soil reinforcement options, mechanically stabilized embankments, and combination structures all require large volumes of earthworks in addition to comparatively expensive and time consuming installation methods.
Steel sheet piles have also traditionally been used to stabilize slopes and grade separation structures. The sheet piles are driven into the ground from the ground surface and pass through both the yielding unstable ground and downward into the stable ground. The upper portion of the sheet piles captures the driving forces; the lower portion transfers driving forces to the adjacent soil; and the portion of the sheet pile that extends through the interface of the stable and unstable soil horizons transfers the driving shear load and bending moments to the lower resisting portion of the section. Although the system is robust, it is also expensive because of the high cost of steel that is used in the sections. For this reason, they are seldom used.
U.S. Pat. Nos. 7,090,440 and 7,811,032 issued to Short deal with stabilizing shallow failing slopes with plate piles consisting of a unitary steel plate affixed to a pile stem. Each plate pile is configured to “catch” a volume of downslope-moving soil within the sliding soil mass. The pile stem transfers the applied load into the underlying competent materials. The plate pile system marks an improvement over steel sheet piles because the plate piles are installed a few feet on-center allowing for the use of less steel and a more efficient pile stem section to transfer the applied shear loads and bending moments. The system, however, was developed for installation in soil profiles characterized by weathered residual soil overlying competent very stiff soil and soft rock wherein the thin pile stem is small enough to be easily driven into the hard lower materials and wherein the pile stem transfers its load efficiently into these hard underlying materials. However, a shortcoming of this system is that it does not efficiently transfer load into underlying stable medium stiff soil layers commonly associated with levees and other earthen structures.